--- a/sys/sys/libkern.h
+++ b/sys/sys/libkern.h
@@ -64,7 +64,7 @@
 /*
  * Number of bytes needed for hexadecimal representation
  * plus trailing \0
  */
-#define	HEX_NCPYLEN(s)	(s * 3)
+#define	HEX_NCPYLEN(s)	((s) * 3)
 
 static __inline int imax(int a, int b) { return (a > b ? a : b); }